The present invention relates generally to thin film optical filters, and more particularly to a method for fabricating, in a single step, a gradient index optical filter having a specified reflectance function and constrained refractive index contrast.
Thin film optical filters find substantial application in filtering invasive laser light to prevent damage to biological and electrical sensors. The iterative structure of thin-film optical filters is described, for example in Thin Film Optical Filters, by H. A. Macleod (McGraw-Hill Publishing Co., New York (1989) the entire teachings of which are incorporated herein by reference.
Gradient index or rugate optical filters are continuous generalizations of the familiar quarterwave stack which find widespread applications in sensor hardening. In the design of a filter or mirror there are infinitely many refractive index profiles n(x) whose wavelength-dependent reflectance R(.lambda.) is specified as a design objective. The relationship between R(.lambda.) and the refractive index n(x) is nearly that of a Fourier transform pair. In accordance with a governing principle of the present invention, exploiting this relationship, specifically the physically indeterminate phase of the Fourier transforms, permits constrained optimization of rugate designs. Accordingly, the invention provides a novel method for fabricating a rugate type gradient index optical filter, structured as a thin film optical coating having a preselected reflectance as a function of wavelength, in which the refractive index is a continuous function of filter thickness. A filter fabricated according to the invention is therefore distinct from conventional quarter-wave filters in which alternate layers of materials define a discontinuous refractive index function through the thickness of the filter.
It is therefore a principal object of the invention to provide an improved optical filter.
It is a further object of the invention to provide a method for fabricating a thin film gradient index optical filter.
It is a further object of the invention to provide a gradient index optical filter with desired reflectance functions and constrained refractive index contrast in a single step.
It is a further object of the invention to provide a method for fabricating a thin filter having low refractive index contrasts.
It is yet a further object of the invention to provide a method for fabricating a thin film optical filter having a continuous sinusoidal refractive index function.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.